Method for triggering a thermostat

ABSTRACT

The invention relates to a cooling system with pulse-width triggering for the operating elements on the valves of the thermostat being subjected to closed-loop control in an adaptive manner. The aim is to reach the required temperature level in the coolant circuit as quickly as possible initially by predetermined and stored basic adaptation, taking into account the current ambient temperature. Depending on the load state and ambient conditions, three different temperature levels are provided as desired variables for setting the coolant temperature. Once the currently required coolant temperature is reached for the first time after starting, closed-loop control is changed over to fine adaptation. The coolant temperature which is currently to be set is kept as constant as possible by fine adaptation as a function of the desired temperature and the external temperature. If the desired temperature of the coolant, which temperature is to be achieved by closed-loop control, changes on account of a change in the load state of the engine, the newly required temperature level is set by fine adaptation. This has the advantage that, when the motor vehicle is started, the coolant temperature which is currently to be set can be achieved immediately by the basic adaptation settings.

The invention relates to a method for triggering a thermostat, inparticular in a cooling system of a motor vehicle.

A cooling arrangement, which forms this generic type, and a method foroperating the cooling arrangement, which forms this generic type, areknown from DE 44 09 547. This cooling arrangement can be used to set twodifferent coolant temperatures as a function of specific operatingparameters of the vehicle. The influencing operating parameters in thiscase are the vehicle speed, the load state of the internal combustionengine, and the intake air temperature. As a function of theabovementioned parameters, a control algorithm is used to decide whichtemperature level the coolant should be set to. In this case, thethermostat in the cooling circuit is triggered by a controller in whichthe control algorithm is implemented. The temperature levels providedare 90° Celsius and 110° Celsius.

The abovementioned two-point closed-loop control systems tend tooscillate. This problem always occurs when the influencing variables andtheir values are in a value range in which the control algorithm is setto the respectively other temperature level when there is an extremelysmall change in the influencing variables. In addition, methods whichare already known do not take into account the external temperature,that is to say the ambient temperature, even though the ambienttemperatures may fluctuate greatly and have a great effect on the enginetemperature and the possible cooling power of the cooling system inextreme weather conditions.

The problem of oscillation has already been identified in EP 0 744 538A2. The solution proposed is adaptive closed-loop control. The proposalmade is that of evaluating the system response to a jump function andusing this to adapt the control parameters in an adaptive fashion.

The object of the invention is therefore to specify a method fortriggering a thermostat, which method does not tend to oscillate andalso takes into account the ambient temperature.

The object is achieved by the features of claim 1. Advantageousembodiments of the invention can be found in the subclaims and in thedescription of the exemplary embodiments.

The solution is achieved mainly by pulse-width triggering for theoperating elements on the valves of the thermostat being subjected toclosed-loop control in an adaptive manner. The aim is to reach therequired temperature level in the coolant circuit as quickly as possibleinitially by predetermined and stored basic adaptation, taking intoaccount the current ambient temperature. Depending on the load state andambient conditions, three different temperature levels are provided asdesired variables for setting the coolant temperature. Once thecurrently required coolant temperature is reached for the first timeafter starting, closed-loop control is changed over to fine adaptation.The coolant temperature which is currently to be set is kept as constantas possible by fine adaptation as a function of the desired temperatureand the external temperature. If the desired temperature of the coolant,which temperature is to be achieved by closed-loop control, changes onaccount of a change in the load state of the engine, the newly requiredtemperature level is set by fine adaptation. This has the advantagethat, when the motor vehicle is started, the coolant temperature whichis currently to be set can be achieved immediately by the basicadaptation settings.

Fine adaptation is used for adjustment purposes if the coolanttemperature set by basic adaptation deviates from the desiredtemperature. The settings obtained by fine adaptation are, in this case,stored at regular intervals of, for example, 100 seconds, and the basicadaptation settings are overwritten by the new settings. In this way,basic adaptation is matched to the currently prevailing ambientconditions and to the driving style of the driver of the motor vehicle.In this case, the new settings are determined separately and storedspecifically for each of the three prespecified temperature levels of80° C., 90° C. and 105° Celsius. Basic adaptation therefore respectivelycomprises settings for the temperature level of 80° C., for thetemperature level of 90° C., and for the temperature level of 105° C.

In one advantageous embodiment of the invention, the stored basicadaptation settings are matched to the ambient temperature by means of acorrection function. This correction is made whenever the ambienttemperature has changed by a prespecified temperature interval of, forexample, 8° Celsius and if the motor vehicle has been out of operationfor a prespecified minimum time period of, for example, 2 hours. In thiscase, the correction is made immediately when the vehicle is restarted,even before basic adaptation begins. Basic adaptation therefore alreadybegins with corrected settings when the ambient conditions have changedconsiderably, for example if the vehicle was turned off overnight, withthe result that it is not necessary to first find new settings by fineadaptation. This advantage is important when, for example, the motorvehicle has been switched off on a hot day and is operated again on afollowing, cooler day. In this case, contrary to other adaptiveclosed-loop control systems, for example in EP 0 744 538 A2 whichemploys the control parameters used last, in the method according to theinvention, basic adaptation begins with the adapted control parameters,so that it is not necessary to first find new control parameters for thenew ambient conditions.

A further advantage of preset basic adaptation is given with the use ofa motor vehicle in different climate zones. In this case, the coolingarrangement of the vehicle can be matched to the respective climate zonein an optimum manner by basic adaptation which is geared toward therespective climate zone. The daily temperature fluctuations and thevariable load conditions of the engine are compensated for by fineadaptation.

In one advantageous embodiment, the method according to the inventionhas a fallback level such that control of the coolant is taken over by aproportional controller if the two adaptation stages fail.

A further advantage of the method according to the invention is seen inthe ability, in contrast to the prior art, to set three differenttemperature levels for the coolant temperature. This has the advantagethat the engine temperature can be matched more effectively both to theambient conditions and to the load state of the engine.

Exemplary embodiments of the invention are explained below in greaterdetail with reference to figures, in which:

FIG. 1 schematically shows a cooling system with the influencingvariables which are most important for the invention;

FIG. 2 shows a simplified Matlab-Simulink representation for determiningthe temperature level to be set; and

FIG. 3 shows a simplified Matlab-Simulink representation of the adaptiveclosed-loop control system.

FIG. 1 schematically shows a typical cooling system for a six-cylinderinternal combustion engine 1. In addition to the internal combustionengine, a vehicle radiator 2 and a heat exchanger 3 are integrated inthe cooling system. The cooling power of the vehicle radiator can beinfluenced by an electrically driven fan 4. In order to regulate thepower of the fan, the electric motor of the fan is subjected toclosed-loop control by a control device 5. Cooled coolant is taken fromthe vehicle radiator by means of the feed line 6 and fed to the coolinglines 8 by the coolant pump 7 in order to feed the cooling channels (notillustrated in any detail) for the combustion cylinders 9. The heatedcoolant is passed from the combustion cylinders 9 to a three-waythermostat 11 via return lines 10. Depending on the position of thevalves in the three-way thermostat 11, the coolant passes out of theinternal combustion engine and back again into the vehicle radiator viathe radiator return 12, or back again into the cooling lines 8 of theinternal combustion engine via the radiator short-circuit 13 and thecoolant pump 7.

Depending on the position of the valves in the three-way thermostat 11,the cooling system may be operated here, in a manner known per se, inthe short-circuit operating mode, in the mixed operating mode or in thelarge cooling circuit. The heat exchanger 3 is connected to thehigh-temperature branch of the cooling system in the internal combustionengine via a temperature-controlled shut-off valve 14. The throughputthrough the heat exchanger after the shut-off valve 14 is opened can beregulated with an additional electric coolant pump 15 and a pulsedshut-off valve 16 in order to regulate the heating power.

The operating elements on the valves of the three-way thermostat 11 aretriggered here by the control device 5. The control device contains alogic component, Logic, in the form of a microelectronic computer unit.The control device is preferably formed by the controller of the engineelectronics system. The control algorithms which are outlined in FIGS. 2and 3 are implemented in the logic component in the form of softwareprograms. In this case, the most important operating data for adaptationof the control parameters are: the actual coolant temperature, thedesired coolant temperature, the external air temperature, the PWM pulseduty factor for triggering the valves, and a fault detection means,Failsafe, for activating a fallback level when the closed-loop controlsystem fails.

FIG. 2 shows a simplified Matlab-Simulink representation of the softwarearchitecture and the signal flowchart for determining, according to theinvention, the coolant temperature to be set. The input signalscomprising the intake air temperature 21, mass air flow 22,classification 23 of the driver type, engine speed 24, fuel injectionquantity 25 and external air temperature 26 are further processed with afive-stage decision cascade, and the desired coolant temperature 27which is matched to the current operating parameters is determined fromthis. Each stage of the decision cascade is composed of an EDP programfor deciding on and calculating a desired temperature as a function ofthe program input variables. The individual software programs arereferred to below as modules.

Here, in engines with port injection, the five-stage decision cascade iscomposed of the modules KE_ECT (for KanalEinspritzer [port injector]Engine Cooling Temperature), ECT_FTK (for Engine Cooling Temperatureaccording to Fahrertypklassifizierung [classification of driver type]),ECT_AT (for Engine Cooling Temperature according to Ansauglufttemperatur[intake air temperature]), ECT_VehSpd (for Engine Cooling Temperatureaccording to Vehicle Speed) and the module ECT_ExtAir (for EngineCooling Temperature according to External Air Temperature).

In engines with direct injection, the quantity of fuel is determinedfrom the injection quantity. In these engines, the module DE_ECT (forDirekt Einspritzung [direct injection] Engine Cooling Temperature) isused instead of the module KE_ECT for calculating a first desiredcoolant temperature TMSoll1. The control algorithm contains bothmodules, for the port injector as well as for the direct injector, asstandard. Which module is used is set on an engine-specific basis byactivating one of the two modules by means of a program. This choice isrepresented in the signal flowchart according to FIG. 2 by the switchingelement 28. This procedure has the advantage that only one controlalgorithm has to be implemented for the various types of mixtureformation, and said algorithm can then be set to the respective engineversion.

The first desired coolant temperature TMSoll1 which is calculated fromthe fuel input is load dependent, that is to say is set to 105° Celsiusor to 80° Celsius as a function of the engine speed EngSpd and thequantity of fuel. The first desired coolant temperature TMSoll1 isweighted using the following module ECT_FTK as a function of the currentclassification FTK of the driver type from the engine controller andeither a coolant temperature of 105° Celsius or of 80° Celsius isselected in accordance with the classification of the driver type. Thecoolant temperature of 80° Celsius is weighted more heavily, i.e. isselected with preference, for classification of the driver type assporty. The result of this weighting is a second desired coolanttemperature TMSoll2.

After the classification of the driver type, the intake air temperatureis taken into account in the next stage of the decision cascade. This isdone in the module ECT_AT. Detection of the intake air temperatureserves to identify a traffic jam. If the motor vehicle is stuck in atraffic jam, it is desirable to lower the desired coolant temperature to80° Celsius or 90° Celsius, which is triggered by this traffic jam. Thisis done by lowering the coolant temperature to one of the twoabovementioned values if the intake air temperature exceeds a referencevalue from the temperature interval 40° Celsius to 50° Celsius. Theresult, after taking into account the intake air temperature, is thedesired coolant temperature TMSoll3.

This desired coolant temperature TMSoll3 which is determined isevaluated in the decision cascade by means of the next module ECT_VehSpdusing the current vehicle speed. If the vehicle speed exceeds a firstreference value, for example 120 km/h, the coolant temperature is set to90° Celsius, and if the vehicle speed exceeds a second reference value,for example 160 km/h, the desired coolant temperature is set to 80°Celsius.

In the last stage of the decision cascade, the desired coolanttemperature TMSoll4 which is evaluated according to the vehicle speed isevaluated and determined using the external air temperature. In thisway, the previously obtained desired coolant temperatures can ultimatelybe overridden in extreme environmental conditions, for example extremecold, and a desired coolant temperature TMSoll5 which is to beultimately applied can be determined, said desired coolant temperatureTMSoll5 being predefined as a desired variable for the triggering meansof the fan 4 and the three-way thermostat 11. If the externaltemperature exceeds a first reference value of, for example, 12°Celsius, the temperature is not lowered by the last stage of thedecision cascade. The desired coolant temperature is adapted to theexternal temperature when the temperature falls below the firstreference value, of for example 12° Celsius, to a desired coolanttemperature of 90° Celsius. If the external temperature drops furtherand if it falls below a second reference value, of for example minus 15°Celsius, the desired coolant temperature is set to 105° Celsiusindependently of the other influencing variables.

The desired coolant temperature TMSoll5 which is ultimately presentafter the fifth stage is retained as a desired variable for the adaptiveclosed-loop control system according to FIG. 3 for a minimum timeperiod, of for example 100 seconds, independently of the input signals21, 22, 23, 24, 25, 26 and of the vehicle speed. This hold function canbe realized, for example, with a holding element or a program waitingloop. In the signal flowchart in FIG. 2, the hold function of thedesired coolant temperature which is determined is symbolized by atiming holding element 29.

The desired coolant temperature which is determined by the decisioncascade according to FIG. 2 is finally further processed by the adaptiveopen-loop and closed-loop control systems, as outlined in greater detailin FIG. 3. The input end of the open-loop control system is providedwith signal values for the desired coolant temperature TMSoll, for theactual coolant temperature TMIst, for the external air temperature, forthe basic adaptation open-loop and closed-loop control parameters whichare to be read in, GA Parameters, for the activation of basicadaptation, Activation GA, for the activation of fine adaptation,Activation FA, and for the activation of the fallback level, Failsafe,if the open-loop control system operates incorrectly or fails because,for example, one of the input signals is no longer available. In FIG. 3,the signal input is symbolically represented by the connection pins 31,32, 33, 34, 35, 36 and 37 and denoted with the corresponding signalvalue.

The triggering means for the thermostat 11 comprises the software module40 for basic adaptation, the software module 41 for fine adaptation, thesoftware module 42 for pilot control of the operating elements on thevalves of the three-way thermostat 11, and a digital proportionalcontroller 43 which may also be in the form of a software module.

Basic adaptation is activated when the control device 5 is connected tothe voltage of the vehicle electrical system when the vehicle is startedand the desired coolant temperature is less than 90° Celsius. Thedesired coolant temperature is used as a decision criterion foractivation of basic adaptation so that a check by a (German) technicalsupervisory body of the exhaust gas limit values is not impeded. To-beprecise, engine temperatures of 105° Celsius, which are optimal forexhaust gas levels, are used for the statutorily prescribed exhaust gastest, so that basic adaptation cannot be used for setting a desiredtemperature, which is determined in accordance with the algorithm fromFIG. 2, of below 105° Celsius. In other words, the three-way thermostat11 is not triggered by basic adaptation during the exhaust gas test. Inaddition, basic adaptation must be active only during operation of theengine. If basic adaptation were active when the engine is at astandstill, for example, this would corrupt the adaptation values in theform of basic adaptation values GA_Parameters on the terminal 34 in thecase of self-adaptation of basic adaptation at the predominantlyprevailing ambient climatic conditions.

A self-reset function of the GA_Parameters may be performed, forexample, by the submodule GA_Reset from FIG. 4. This submodule isintegrated in the software module 40 for basic adaptation. The controldeviation between the actual coolant temperature and the desired coolanttemperature is also registered and integrated in this submodule. If theintegral exceeds a specific value, basic adaptation is reset and theoriginal control parameters are replaced by new control parameterswhich, for example, are calculated from the integrated temperaturedeviation and the temperature characteristic diagram of the pilotcontrol means for triggering the thermostat. At the start of theintegral, the actual temperature has to be within the range of theproportional controller 43 once. The reset is used to improve thecontrol parameters of basic adaptation when they are very poor. Thereset also matches basic adaptation to different ambient climaticconditions.

The correction factor TMGACorr for varying the controller parameters ofbasic adaptation is obtained, for example, from the mean integratedtemperature Tmean of the respective desired temperature TMSoll of 80°Celsius or 90° Celsius and the characteristic variable of thepulse-width control TMGAGrad for the change in the cooling watertemperature as a function of the pulse duty factor of the pulse-widthcontrol in accordance with the following equation:TMGACorr=(Tmean−TMSoll)*TMGAGradwhere TMGAGrad is measured in %/° C. A typical value for a pulse-widthcontrol used was a 3% increase in the pulse duty factor for a 1°temperature decrease in the coolant circuit. The desired temperaturevalues which are determined in accordance with the algorithm from FIG. 2can be used for TMSoll. The correction determined in this way is still afunction of the ambient temperature.

The dependence of the settings on the ambient temperature is taken intoaccount by a further correction function which is integrated in thesoftware module 40 for basic adaptation. To this end, the external airtemperature is read in at PIN 33 as a digital value. The effect of theexternal air temperature on the cooling power of the cooling system istaken into account by a correction characteristic diagram and a pulseduty factor of the pulse-width control is accordingly selected, thispulse duty factor compensating for the effect of the externaltemperature. This compensation may involve, for example, taking intoaccount the effect of the external temperature as a multiplicativecorrection factor for changing the cooling water temperature as afunction of the pulse duty factor TMGAGrad. The correction factor canthen expediently be found in the abovementioned characteristic diagramas a function of the measured external temperature.

After the controller has been connected to the voltage of the vehicleelectrical system, basic adaptation is generally active only once forthe following driving cycle. In contrast, fine adaptation 41 (FIG. 3)runs permanently and begins after the desired temperature of 80° Celsiusor 90° Celsius has been reached for the first time by basic adaptationand basic adaptation has ended. By way of example, a threshold valuecomparator (not illustrated) can establish when the desired temperatureis reached and then transmit a corresponding start signal, ActivationFA, to the input pin 36 of the fine adaptation means 41. In contrast tobasic adaptation, correction is determined over time in the case of fineadaptation. The number of time components of the total operating periodof the current fine adaptation phase in which the actual temperature ofthe coolant deviates from the desired temperature is then recorded, forexample. Furthermore, a correction value TMFACorr is calculated in thefine adaptation means 41, fed back to the basic adaptation means 40 in acontrol loop and used to correct triggering of the pilot control means42.

Finally, in the pilot control means 42, the predefined signal TMGA atthe output of the basic adaptation means 40, which signal contains thecorrection information, is used to determine the correction of thepulse-width pulse duty factor in accordance with the characteristiccurves of the operating elements used in the three-way thermostat, andsaid correction is additively superimposed on the controller output ofthe proportional controller 43. The superimposition is symbolicallyrepresented by reference numeral 45 in FIG. 3. The process 30 finallyoutputs a pulse duty factor of the pulse-width modulation which is usedto operate the operating elements in the three-way thermostat.

The closed-loop control system according to FIG. 3 has the advantage, inparticular on account of the adaptive superimposition of basicadaptation and fine adaptation on the output of the proportionalcontroller 43, that an emergency function can be provided in a verysimple manner. If the basic adaptation means or the fine adaptationmeans is not operating correctly, the two modules can be switched off ina simple manner by a corresponding signal, Failsafe, which is symbolizedat terminal 37. The coolant temperature is then set solely by theproportional controller 43.

The ambient conditions are taken into account by detecting the externalair temperature by means of a corresponding temperature sensor whichsupplies a temperature signal to the input of the terminal 33. Thismeasured external air temperature is taken into account by the softwarein the proportional controller 43, by the software in the basicadaptation means 40 and by the software in the fine adaptation meanswhen determining the controller settings and adaptation. Saidtemperature is taken into account here using a computer by means oftemperature characteristic diagrams which take into account thedependence of the cooling power on the external temperature. It is thuspossible to set triggering of the three-way thermostat to the currentambient temperature too. Adaptation can therefore be deactivatedparticularly in the case of high external temperatures which maypossibly prevent a desired coolant temperature of 80° Celsius beingreached, since adaptation would be nonsensical in the case of impossiblepredefined desired values.

1. A method for the closed-loop control of a thermostat (11), inparticular in a cooling circuit of an internal combustion engine (1),wherein, by means of the valves in the thermostat, a small coolantcircuit without a radiator (2) and a large coolant circuit with aradiator (2) can be separated from one another or connected to oneanother in a temperature-controlled manner, or connected to one anotherin a mixing mode with a mixing ratio with closed-loop control of thetemperature, and the operating units of the valves in the thermostat(11) are triggered by a control means (5), and one of a plurality ofpossible desired coolant temperatures is set by opening and closing thevalves in the thermostat, characterized in that closed-loop control toeach prespecified desired coolant temperature involves a first andsecond closed-loop control phase, with the first closed-loop controlphase in the form of basic adaptation (40) with stored controlparameters setting the currently prespecified desired coolanttemperature as quickly as possible, and, after the respectively currentdesired coolant temperature is reached, the second closed-loop controlphase in the form of fine adaptation (41) with variable controlparameters keeping the currently prespecified desired coolanttemperature as constant as possible.
 2. The method as claimed in claim1, characterized in that, when the currently prespecified desiredcoolant temperature is changed, the new desired coolant temperature isset by fine adaptation.
 3. The method as claimed in claim 1,characterized in that the basic adaptation settings are improved by thecorrected fine adaptation settings.
 4. The method as claimed in claim 1,characterized in that, when the motor vehicle is started, the basicadaptation settings are matched to the ambient temperature.
 5. Themethod as claimed in claim 4, characterized in that, when the motorvehicle is started, the basic adaptation settings are adapted if theambient temperature has changed at least by a prespecified temperatureinterval and the motor vehicle has been out of operation for aprespecified minimum period.
 6. The method as claimed in claim 1,characterized in that the current desired coolant temperature (TMSoll)is selected from amongst three different prespecified desired coolanttemperatures as a function of the load.
 7. The method as claimed inclaim 1, characterized in that the external air temperature (33) is alsoentered into the closed-loop control system in the first and in thesecond closed-loop control phase.
 8. The method as claimed in claim 1,characterized in that basic adaptation (40) can be deactivated and,particularly in the event of a fault, closed-loop control of the coolantis taken over from a redundant failback level by a proportionalcontroller (43).